What's The Best Sub-$100 Automotive Upgrade?

Since y'all seemed to dislike the ten daily driver modifications post we syndicated last week, here's your chance to chime in on cheap mods that are actually worthwhile. What's the best sub-$100 automotive upgrade?

There's always some debate as to the effectiveness of a good cone filter in terms of actual power output and fuel economy, but here's a few things we can say for certain about the upgrade: it's cheap (way less than $100), it improves the noise you can actually hear while driving the car (intake), it typically provides superior engine protection, and it's a lifetime part so you don't have to keep buying paper filters to replace it. It also makes your car look just a little special.

It's a bad economy and the people have a fever, a fever for upgrades. Help 'em out.

(QOTD is your chance to answer the day's most pressing automotive questions and experience the opinions of the insightful insiders, practicing pundits and gleeful gearheads that make up the Jalopnik commentariat. If you've got a suggestion for a good "Question Of the Day" send an email to tips at jalopnik dot com.)